I do not think a 13 year old should be dieting. They should be having fun and playing with their mates. How tall are you and what is your size? Acai Berries are supper foods, but you should be having a wide range of foods in your diets and be exercising. It is all about the portions you take. However, I think you are to young to even be worrying about diets! Live life!
I agree that you should not be worried about dieting yet. Dieting so young and not getting all the nutrition you need while you're developing can weaken your immune system too, and create problems for you later in life. At 13 your body doesn't look the way it will when you get older anyway. I know mine didn't. I'm only 20 and I look nothing like I did at 13; I'm actually slimmer, but not for dieting that's for sure! Sometimes your body has to grow into itself. If you're still unhappy with your body when you're more like 17 or 18 then you can diet, but I'd stay away from fad diets like the acai berry diet. They usually don't produce long lasting results.
If you just wanted to be healthy now you could always try small steps like replacing soda with juice or water, or skipping dessert. It's pretty safe to say that you're not losing any nutrition there. lol! You'd be suprised how such little things will help out.